By choosing Calendar dates you can sort files according to the date field specified at the bottom of the calendar pane. The date fields you can choose between are Modified date, Camera date, Created date, Created date (IPTC) and Release date (IPTC).
If a date in the calendar is in bold type, then one or more files will have that date in the date field selected. To display files within a date interval, you can click and drag a range of dates in the calendar.
Below the calendar(s) you find a tree view. This view gives you information about the files in the currently selected source. You will for example see the number of files for a specific month.